docs: def _get_alpha(self, C, vs30, pga_rock):
          """
          Returns the alpha, the linearised functional relationship between the
          site amplification and the PGA on rock. Equation 31.
          """
          alpha = np.zeros(len(pga_rock))
          idx = vs30 < C["k1"]
          if np.any(idx):
              af1 = pga_rock[idx] +\
                  self.CONSTS["c"] * ((vs30[idx] / C["k1"]) ** self.CONSTS["n"])
              af2 = pga_rock[idx] + self.CONSTS["c"]
              alpha[idx] = C["k2"] * pga_rock[idx] * ((1.0 / af1) - (1.0 / af2))
          return alpha